1. Field of the Invention
The present invention relates to a wireless IC device used for an RFID (Radio Frequency Identification) system for performing wireless data communication using electromagnetic waves.
2. Description of the Related Art
Recently, as a product management system, an RFID system has been used in which a reader/writer arranged to generate an induction field communicates with a wireless IC device attached to a product in a wireless manner so as to obtain predetermined information stored in the wireless IC device.
FIG. 1 is a diagram illustrating an example of a wireless IC tag (RFID tag) disclosed in Japanese Unexamined Patent Application Publication No. 2005-244778 in which an IC tag label is attached to an IC tag antenna.
In a wireless IC tag T0, a pair of main antenna elements 81, an auxiliary antenna element 82, and a pair of matching portions 83 are provided on the surface of a dielectric substrate 84.
The main antenna elements 81 are meandering antennas in which meandering conducting lines are provided, and are symmetrically arranged on the dielectric substrate 84. Between the main antenna elements 81 occupying areas at both ends of the dielectric substrate 84, the auxiliary antenna element 82 is disposed.
The matching portions 83 are meandering conducting lines (inductors). One end of each of the matching portions 83 is individually connected to an inner end of the main antenna elements 81, and the other end of each of the matching portions 83 is connected to a wireless IC chip 86.
However, the wireless IC tag disclosed in Japanese Unexamined Patent Application Publication No. 2005-244778 has the following problems. Since the matching portions are individually arranged adjacent to the main antennas on the same substrate, the size of the wireless tag is increased.
If the tag is attached to a product having a high dielectric constant, the frequency characteristics of the matching circuit portions are changed due to the influence of the dielectric constant of the product. Accordingly, the frequency characteristic of the tag is significantly changed. If a protection film arranged to cover the surface of a product to which the tag is attached or the surface of the tag, the impedances of the matching portions are changed. Accordingly, it is necessary to design the wireless tag in consideration of the use condition of the wireless tag.
Since the auxiliary antenna is used to increase the design flexibility of the main antenna elements, the size of the tag is increased. Since matching design is performed at portions other than the matching portions, the number of design parameters is increased and the design complexity of the tag is increased.
Since the IC chip must be mounted on a small mounting electrode on a large substrate on which the main antennas and the matching portions are disposed, a high-precision mounting apparatus is required. Since the mounting position adjustment requires a long period of time and the manufacturing time for the tag therefore is increased, the cost of the tag is increased.
Since each of the main antennas is connected to the IC chip so that the DC continuity between them is achieved, static electricity may flow from the main antenna into the wireless IC chip and break the wireless IC chip.